Is it planted in ericaceous compost? If not, that may be the cause. They like acid soil.

Which particular Pieris do you have - have you a label on it still? Because P. japonica varieties are fine in sun, but P. formosa varieties (such as P. formosa 'Forest Flame') much prefer partial shade. They all, though, require neutral to acidic soil, as Ojibway mentions.

Try giving it some ericaceous feed or some sulphate of iron. If the problem is the soil pH this should green it up fairly quickly as long as it isn't really alkaline. You will need to do it a couple of times a year.